1. Field of the Invention
This invention relates to RF probes for Nuclear Magnetic Resonance spectroscopy and microscopy. More particularly, it relates to resonators for the transmission and reception of NMR signals using a plurality of thin-film tuned coils without ohmic interconnection. More particularly, it relates to pairs of tuned, inductively-coupled RF coils fabricated from a superconducting material.
2. Description of Related Art
In an NMR spectrometer probe, a sample is placed in a static magnetic field, which causes atomic nuclei within the sample to align in the direction of the field. Transmit and receive coils, which may be combined in a single coil or set of coils, are placed in the probe positioned close to the sample. The transmit coils apply an RF magnetic field orthogonal to the direction of the static magnetic field, perturbing the alignment of the nuclei. The transmit signal is then turned off, and the resonant RF signal of the sample is detected by the receiver coil.
The sensitivity of the spectrometer depends on a number of factors, including the strength of the static field, the closeness of the coupling between the RF coils and the sample, and the resistance of the RF coil. Currently, all commercial NMR spectrometers use RF coils made of a normal metal, such as copper, or a combination of normal metals. Much research has been devoted to the design of coils for maximum sensitivity. For example, to achieve close coupling, coils have been made in the form of solenoids, saddle coils and birdcage coils, all of which have very high filling factors. Similarly, researchers have suggested cooling of RF coils to reduce their resistance. However, the sensitivity of conventional normal-metal coils is limited by their resistance, even at low temperatures.
The use of high temperature superconductors (HTS) in place of conventional normal metal for RF coils in NMR spectrometers has previously been suggested. For example, Marek U.S. Pat. No. 5,247,256, describes several RF receiver coil arrangements for NMR spectrometers using superconducting coils. Marek's embodiment differs from the present invention in several respects. In particular, Marek's coils are nonplanar and use ohmic contacts.
The advantage to be obtained with HTS coils is significant. HTS coils would have very low resistance and be operable in the range of 20-100 K., temperatures achievable with currently available refrigeration systems. The quality factor Q of the coil is a useful measure of the coil's efficiency. Q=.omega.L/r, where .omega. is the resonant frequency, L is the inductance and r is the resistance of the coil. Well designed normal-metal NMR coils achieve Qs of about 250. Because of the extremely low resistance of HTS coils, it should be possible to design coils with Qs of 10,000 or more. However, this advantage can only be realized if the other factors necessary for a superior NMR probe, reasonable filling factor and high RF and DC field homogeneity, are met. Thus, the ideal RF probe for NMR would have a transmit/receive coil which would resonate at the desired operating frequency, produce a homogeneous RF field, not significantly disturb the DC field, have a high filling factor (and hence high signal to noise ratio), have a high Q, small parasitic losses and produce a high RF magnetic field at the center of the sample.
Thin-film HTS coils offer design challenges not present with normal-metal coils. The high temperature superconductors are perovskite ceramics which require a well-oriented crystal structure for optimum performance. Such orientation is extremely difficult to achieve on a curved substrate. Thus, thin-film HTS coils are preferably planar, making the achievement of a high filling factor more challenging. It is well known in the art that forming ohmic contacts between an HTS and a normal metal is difficult and generally leads to parasitic losses at the point of contact. Additionally, to the extent a normal metal is used in the coil, resistive losses in the metal elements would lessen the advantages gained from the use of the HTS.
In addition to Marek, others have reported thin-film superconductor RF coils for magnetic resonance applications. For example, Withers U.S. Pat. No. 5,276,398 describes a thin-film HTS probe for magnetic resonance imaging. Similarly, Black U.S. Pat. No. 5,258,710 describes HTS thin-film receiver coils for NMR microscopy. Black discloses several embodiments, including split ring, solenoidal, saddle coils, birdcage coils and coils described as "Helmholtz pairs." Black does not disclose how to form the nonplanar solenoidal, saddle, birdcage or Helmholtz coils in an HTS embodiment.
One possible embodiment using planar components is a pair of parallel coils, one on either side of the sample. When the coils are circular and are separated by a distance equal to half the diameter of each coil, they are known as "Helmholtz pairs." Ordinarily, the coils of a Helmholtz pair are ohmically connected using a wire. They may also be capacitively coupled, as in U.S. Pat. No. 5,258,710. An inductively coupled Helmholtz pair is disclosed in M. B. Banson, et al., "A probe for specimen magnetic resonance microscopy," Invest. Radiol. 27, 157 (1992). They report a design for an NMR microscope coil that uses an inductively-coupled copper Helmholtz pair to obtain a uniform magnetic field in the region of the sample.